A method and device of this type are known from the article “Ein Fahrzeugsicherungssystem ohne mechanischen Schlüssel” [A Motor Vehicle Security System without Mechanical Key] by Ch. Schneider and U. Schrey which was published in “Automobiltechnische Zeitschrift 96” (1994), No. 5, as well as from the article “Smart-Card—Abschied vom Autoschlüissel” [Smart-Card-Farewell to the Car Key] by U. Schrey, Ch. Schneider and M. Siedentrop which was published in “Siemens Zeitschrift 1/96.” The access control device described in these articles essentially consists of a transceiver device that is assigned to the motor vehicle and a mobile transponder. The transceiver device comprises three transmitters (LF-transmitters) that operate in the inductive frequency range, where one transmitter is assigned to the driver-side door, another is assigned to the passenger-side door and a third is assigned to the trunk of the motor vehicle. The transmitting antennas of the LF-transmitters are integrated into the respective car doors and into the rear bumper. Each LF-transmitter is connected to the door handle assigned to the respective door and to the trunk release button in such a way that when the door handle or the trunk release button is actuated, the LF-transmitter assigned to this region of the motor vehicle transmits a low-frequency code signal (LF-signal). The remaining LF-transmitters remain inactive and do not transmit LF-signals.
The transceiver device assigned to the motor vehicle is also equipped with an HF-receiver for receiving an HF-signal from the transponder. The answer signal of the transponder that is received in the HF-receiver acts on a control unit that is equipped with a processor. The answer signal that was received in the form of an encrypted code is decoded and checked with respect to its validity in this control unit. If the result of this validity check is positive, a corresponding control signal is delivered to the respective door locking mechanism in order to unlock the door, the door handle of which was previously actuated.
The transponder consists of a so-called access card with an LF-receiver and a low-power HF-transmitter. The inductive reception antenna is realized in the form of a frame antenna, and the high-frequency transmission antenna is realized in the form of a printed-circuit board antenna. The LF-signal received by the LF-receiver of the transponder is decoded in the transponder. A microprocessor assigned to the transponder then processes the code word contained in the LF-signal with a security algorithm and transmits the result to the HF-receiver of the transceiver device assigned to the motor vehicle by means of the HF-transmitter.
The transmission of LF-signals for “awakening” the transponder results in a limitation of the functional radius due to the rapidly decreasing magnetic field. Reception of the transmitted LF-signal is only possible within a functional radius of 1.5-2 m. This is intended to ensure that only those transponders actually located within such a close vicinity are awakened by the LF-transmitter. A definitive localization of the transponder or the person carrying the transponder is effected due to the required actuation of a door handle or a trunk release button and the limited functional radius.
Modified door handle and lock mechanisms that make immediate access to the automobile possible via the door, the handle of which was previously actuated, are used for unlocking the respective motor vehicle door as rapidly as possible. However, even these mechanisms do not make it possible to shorten the time required for transmitting the data from the LF-transmitter to the transponder. This applies, in particular, if complex encrypted codes must be transmitted on this low-frequency link because this process requires a certain time due to the low data transmission speed. However, the transmission of the decoded answer signal from the transponder to the HF-receiver only delays the beginning of the door unlocking process insignificantly.
Another keyless access control device for motor vehicles is known from DE 43 29 697 C2. This device uses a central LF-transmitter for transmitting the low-frequency code signal. However, the LF-transmitter of this device transmits LF-signals cyclically within certain intervals. The transponder is only awakened and transmits its answer signal when the transponder assigned to the motor vehicle is located within the functional radius of the LF-transmitter. In this device, actuation of a door handle or a trunk release button is not required to trigger the unlocking mechanism. Since with this device localization of the transponder in order to unlock only certain doors is not possible, this access control device is only suitable for motor vehicles with a centrally controlled locking system. In this known access control device, a delayed unlocking also occasionally occurs due to the transmission time of the LF-signal that may have a complex design under certain circumstances.
DE 197 28 761 C1 also describes a keyless access control device that uses several LF-transmitters. These LF-transmitters are arranged within certain regions of the motor vehicle and each transmits a signal that differs from those from the other LF-transmitters. The mobile transponder that contains an LF-receiver and an HF-transceiver device is awakened by the LF-transmitter and subsequently begins a question-and-answer dialogue with the transceiver device assigned to the motor vehicle in order to check access authorization. If the result of this question-and-answer dialogue is positive, the corresponding motor vehicle door is unlocked.
The advantage of this keyless access control device is that it is not necessary to actuate a door handle in order to initiate communication between the motor vehicle and the transponder, i.e., no noticeable delay occurs between the actuation of the door handle and the unlocking of the door. However, this device also has the disadvantage that the door(s) may be unintentionally unlocked if an authorized user is in the vicinity of the motor vehicle.